The invention relates to digital tape drive storage devices, and in particular, to a lateral tape motion sensor that detects and responds to lateral movement of multitrack tape media.
It is a problem in the field of tape drives to maintain alignment of the tape drive head assembly and the track as data is being transferred to or from the magnetic tape media as the magnetic tape media is wound between the supply reel located with the tape cartridge and the take-up reel located within the tape drive while providing a plurality of narrow tracks on one length of tape media. Providing a plurality of narrow tracks on one length of tape media for recording data increases tape storage densities. To maximize capacity, tracks are recorded as closely as possible to adjacent tracks. It is a problem in the field of tape drives to prevent overwriting data previously written on an adjacent track when the magnetic tape media moves laterally.
Lateral tape motion (commonly referred to as LTM) is movement of the tape media in a direction perpendicular to the tracks. Lateral tape motion is the tendency of tape media to move laterally due to poor quality tape media, excessive usage of a tape cartridge, excessive vibration or movement of the tape drive during operation, wobbling of the supply or take-up reel, or by imperfect tape guides. It is common practice in the industry to discard tapes after a plurality of uses to prevent inadvertent erasure of data on adjacent tracks due to LTM.
Apparatus are known for adjusting the position of the tape head in response to lateral movement of the tape media and for maintaining the alignment of the tape media as data is transferred to and from the tape media. These devices include apparatus to prevent overwriting data on an adjacent track by detecting lateral tape motion and quickly moving the tape head in the same direction to compensate for the movement. However, precise measurement of the tape motion and a fast mechanical response is required, making such systems complex and therefore costly solutions to a common problem.
Read After Write System:
Miller, (U.S. Pat. No. 4,414,593) discloses a serpentine tape drive apparatus for writing and reading data on a magnetic tape. Using a write-read magnetic head, information is written onto the tape and subsequently read and checked in a continuous manner to assure the correctness of the data written onto the tape. In a serpentine tape drive, the tape moves past the head in a first direction, and when the end of the tape is reached, the head is adjusted to a new track and the tape travels past the head in an opposite direction as the write-read operation continues. Writing and reading as the tape moves in opposite directions provides adjacent serpentine tracks. Reading the data immediately after a write verifies that the data written is correct. The read after write system described in the Miller patent does not check adjacent tracks as data is being written to prevent the tape head from writing over data on an adjacent track.
Track Following Systems:
Apparatus are known for adjusting the magnetic head while reading and writing to maintain alignment between the head and the edge of the tape. The track following servo apparatus disclosed by Saliba, (U.S. Pat. No. 5,371,638) reduces cross talk and the effects of lateral tape motion by adjusting the position of the magnetic head during read and write operations. The apparatus includes a magnetic head with longitudinally placed read and write head gaps and a servomotor for adjusting the head. The read gaps are laterally offset from the write gaps and data is read immediately after a write to check for errors. If the read does not correspond to the write, the data is rewritten. The recorded data includes both low frequency servo blocks and higher frequency data. As a track is written, the adjacent tracks are monitored for low frequency servo data. When servo data is encountered, the write is temporarily suspended. The servo data is compared with predetermined reference data to determine a corresponding head adjustment. The reference data may be last recorded head positional information. The tape head is adjusted corresponding to the servo data in relation to the predetermined reference data. If the tape head is adjusted, a new positional reference is created and stored. The embedded blocks of servo data are also used during reading.
While this system, referred to in the industry as a track following servo system, reduces the risk of overwriting data on adjacent tracks, it requires a costly, precise mechanism capable of moving the tape head as quickly as the tape media moves. Providing a complex apparatus with an additional read/write head that quickly responds to lateral tape motion is expensive and not suitable for use with slower responding tape head designs. The positioning apparatus for adjusting the magnetic head during reading and writing disclosed by Fasen, (U.S. Pat. No. 5,999,359) uses a light source that moves with the head to monitor the distance between the head and the edges of the tape media. The light source illuminates the light detector that generates a motor drive signal for adjusting the position of the tape head as necessary.
This system measures the distance between the head and the edge of the tape to obtain and maintain alignment. When the distance between the head and the edge of the tape varies, the tape head is adjusted. The head adjustment is in reference to the lateral edge of the tape without taking into consideration the adjacent tracks. The apparatus disclosed by Fasen employs an expensive tape head design that can move fast enough to follow the signal from the light source. While the apparatus adjusts the tape head to compensate for lateral tape motion, like the track following servo apparatus previously described, the apparatus is complex and therefore expensive.
Another track following apparatus utilizing a light source for positioning the tape head is disclosed by Iwamatsu, (U.S. Pat. No. 5,196,969) and includes a feedback control apparatus for precisely positioning the head. The feedback control comprises a light emitting source located on one side of the magnetic tape and a plurality of light receiving devices located on the opposite side of the tape and directly across from the light emitting device. The light receiving devices are positioned relative to tracks and with respect to each other to provide tape position feedback. The plurality of light receiving devices has a predetermined relation that governs the feedback control when the magnetic head is moved to a predetermined track position. When the feedback from the plurality of light receiving devices varies from the predetermined track position, the head is adjusted correspondingly.
As the tape head moves widthwise to an adjacent track, the light receiving device corresponding to that track should be illuminated by the light emitting device. Similarly, as the head is reading or writing data, the system monitors the light received by the plurality of light receiving devices to determine the alignment of the head with respect to the tape. Data received from the light receiving devices is compared to the predetermined data, and if a misalignment is encountered, the tape head is adjusted accordingly.
The track following apparatus described above provide an apparatus and method for responding to lateral tape motion, however, the apparatus must have the capability of moving fast enough to follow the vertical tape motion because the tape is so light that it can move very quickly. Apparatus that monitor the lateral position of the tape media and quickly adjust the tape head to compensate for the movement are therefore complex, expensive and are known to be prone to error. The described track following apparatus are not suitable for less expensive, slower responding tape heads.
Tape Aligning:
Another method for aligning tape media with the tape head disclosed in Cope, (Pat. App. No. 20010002158) comprises a series of guide rollers to maintain the alignment of the tape media as it travels past the tape head. One set of guide rollers have a tapered contact surface that applies an upward force on the tape media causing it to maintain constant alignment with the top edge of the guide rollers. As the tape media varies during winding the upward force holds the tape media against the top edge of the guide rollers, which in turn realigns the tape track with the tape head. When the tape media drifts laterally, the rollers realign the tape media keeping the tape media aligned with the tape head. Sudden lateral tape movement realigns over a short period, however, the system does not provide a means to prevent writing data on an adjacent track in the while the tape is realigning.
The read after write system continuously verifies the correctness of the data written, however, the read after write systems offers no protection whatsoever against vertical movement of the tape media The previously described track following apparatus provide a means for preventing writing on a adjacent track when the tape media experiences lateral movement. However, known apparatus that measures the lateral tape motion with a sensor and use fast head mechanisms to follow the motion are complex, costly and are known to be prone to error.
For these reasons, a need exists for a lateral tape motion detector for use with less expensive, slow responding tape head to prevent the accidental erasure of data when lateral movement exceeds a predetermined threshold.
The present lateral tape motion sensor overcomes the problems outlined above and advances the art by providing a sensor that continuously monitors the tape media position and responds when the change in position exceeds predetermined thresholds for a tape drive having a tape head that is not capable of moving vertically as fast as the tape media. A first advantage of the present lateral tape motion sensor is to prevent overwriting data previously recorded on an adjacent track during a write operation. A second advantage of the lateral tape motion sensor is to reduce the time required to search for data during a read operation following a lateral movement of the tape media. A third advantage of the lateral tape motion sensor is to warn users of poor tape quality.
An embodiment of the present lateral tape motion sensor (hereinafter referred to as LTM sensor) comprises a single edge sensing apparatus for continuously monitoring an edge of the tape media as the tape media is wound between the supply reel in the tape cartridge and the take-up reel in the tape drive. The single edge sensing device comprises a light emitting source and a light detecting device for monitoring the top edge or the bottom edge of the tape media.
In another embodiment, the lateral tape motion sensor comprises a dual edge sensing apparatus for continuously sensing the position of the tape media top edge and bottom edge and a method for comparing the movement of the top edge with the movement of the bottom edge to determine a corrective action when the difference exceeds a predetermined threshold.
In the dual edge embodiment, the sensing apparatus includes a top edge sensor and a bottom edge sensor each having a light emitting source and a light detecting device. The light emitting sources and the light detecting devices are horizontally parallel and located on opposite sides of the tape media. In another configuration, the sensing apparatus comprises a light emitting source and a corresponding apparatus located on the opposite side of the light emitting source for detecting light falling above and below the top and bottom edge of tape media. In the various configurations, the sensing apparatus is positioned such that a portion of the light emitted from the light emitting source falls on the light detecting device during normal operation. When the tape media moves laterally, the amount of light falling on the light detecting device varies correspondingly and the light detecting device generates a signal proportional to the amount of light falling on the light detecting device.
The dual edge LTM sensor also includes a method for responding to the variation in movement between tape media top and bottom edges. The dual edge LTM sensor senses the direction and the magnitude of the vertical movement. When the difference between the movement of the top edge and the bottom edge exceed a predetermined threshold, the dual edge LTM sensor responds accordingly. A variation in the top or bottom edge position without a corresponding variation in the opposite edge position, may be the result of a tear or other abnormality in the tape media, and the dual edge LTM sensor assembly may disregard the variation. When approximately equal and opposite variations occur in the top and the bottom tape edge positions, the variation is compared with predetermined thresholds. Variations exceeding the threshold require a system response depending on whether the tape drive is performing a read or a write operation.
Read Operation
When the single edge and dual edge LTM sensors responds to a variation in the lateral position of the tape media during a read operation, the system determines whether the read could be read. If the tape drive was able to read the data, no action is necessary. Conventional tape drives stop, rewind a short distance and attempt to re-read the data following an unsuccessful read. An unsuccessful read simultaneous with sensed vertical movement of the tape media first results in a re-read. If the re-read is unsuccessful, the single edge or dual edge LTM sensor utilizes the direction and magnitude of the movement to determine the approximate location of the data to be read and the magnetic head position is adjusted accordingly. The present single edge and dual edge LTM sensors eliminate the need to randomly search for the data by changing the head position slightly, re-reading the data, and making another head adjustment if the re-read is still unsuccessful. Reducing the time consumed randomly searching for data following lateral movement of the tape improves the overall data throughput.
Write Operation
During a write operation, the present single edge and dual edge LTM sensors protect against the accidental overwrite of data on an adjacent track. When the lateral tape motion exceeds a predetermined threshold during a write operation, the present single edge and dual edge LTM sensors suspend the write operation. The tape drive incorporating the present LTM sensor includes a tape head that does not move vertically as fast as the tape media. Therefore, when lateral tape motion is detected, the LTM sensor signals the tape head control mechanism to suspend a write operation to prevent overwriting data on an adjacent track. The write operation is resumed when the lateral tape movement is no longer detected or the tape head control mechanism has had sufficient time to adjust the tape head. The time between sensing the lateral movement and suspending the write operation is minimal.
In an embodiment of the single edge and dual edge LTM sensors, the drive stops, rewinds and attempts to write the data again. If lateral tape motion is sensed during the subsequent re-write, the write operation is again suspended. In another embodiment of the single edge and dual edge LTM sensors, the write operation is suspended until lateral tape motion is no longer detected, then the write operation is resumed.
Various responses to LTM occurring during a write operation affect the tape drive operational characteristics. Suspending the write operation until lateral movement is not longer sensed increases the throughput while decreasing the storage capacity of the length of tape media. Similarly, while re-writing increases the storage capacity of the length of tape media, the re-write takes time, decreasing the throughput. The responses reside in software which can be configured operate in accord with the various embodiments of the single and dual edge LTM sensors.
Tape Media Quality
In another embodiment of the single edge and dual edge LTM sensors, a plurality of lateral tape motions sensed during one longitudinal pass might be used to indicate poor tape quality or as advance warning of tape degradation due to excessive usage. An indicator located on the tape drive may be illuminated to warn users of the sensed tape media quality.
The present single edge and dual edge LTM sensors operate with less expensive, slow responding tape head mechanisms to reduce or eliminate the undesirable effects of lateral tape motion. The single and dual edge LTM sensors sense vertical movement of the tape media and respond to the movement when it exceeds predetermined thresholds to protect against accidental overwrite on adjacent tracks during a write operation. The single edge and dual edge LTM sensors also provides a method for locating data following excessive lateral movement that occurs during a read operation and can warn the user of poor tape quality. Thus, the single edge and dual edge LTM sensors improve the quality of the data recorded on the tape media and increases the data throughput during read operations without the increased cost associated with a faster responding tape head.